Conventionally, a rotational angle detecting device that detects a rotational angel of a rotating shaft of a motor has been known. For example, a rotational angle detecting device disclosed in a Patent Literature (JP 5339095 B), which is used for an electric power steering device, is powered by electric power from a battery. A controlling unit is disposed in the electric power steering device and calculates a rotational angle of a steering shaft based on a rotational angle of an electric power motor.